This invention relates to conductive adhesives for the thermal interface between a silicon device and a heat sink/heat spreader in microelectronic assemblies. More particularly, this invention is directed to conductive adhesives with improved functional performance and a method to rework the cured adhesives to allow recovery, recycle, or reuse of the heat sink assembly components without causing any detriment to the device chip or the chip carrier.
The rapid technology advancements in high performance electronics packaging has focused on reduced size and higher operating speed. This has resulted in excessive heat generation during device operation. There is an accompanying need for effective heat dissipation methods to maintain the reliable functional performance of electronic assembly products. The commonly used methods of cooling include helium filled modules, solder thermal interfaces, thermal greases, elastomeric silicone gels, thermoplastic polymers with thermally conductive fillers such as AIN, BN, ZnO, and more recently, phase change materials (PCM), and conductive adhesives. These provide the thermal interface between the silicon device chip and a high thermal conductivity metal heat spreader or heat sink to allow a path for heat dissipation from the high power density circuit devices during operation.
Thermal grease is spread as a thin layer between the back of the die and the heat sink. Thermal grease has low thermal resistance and can be easily reworked. However, it is subject to pump-down and drying which causes voids at the interface. This degrades the device performance with time due to an increase in interfacial resistance. The phase change materials (PCM) are low melting waxes. Examples include paraffin wax, having graphite particles dispersed in the wax polymer matrix, and silicone based waxes, such as alkyl methyl silicones, which can be used as preformed tapes or melt dispensed across interfaces. They provide low thermal impedance and high thermal conductivity, typically in the range 5 W/m K in thin bond line thickness. However the pre-cut films of these materials are fragile and also have the problem of performance degradation and variability, delamination, bleed-out, out-gasing, and generally require fasteners, clips or screws to hold the PCM in place.
Another category of thermal interface materials are conductive adhesives which can be used as a thin adhesive interlayer between the heat sink or the heat spreader and the back side of a silicon die in a flip-chip module assembly. The commercially available conductive adhesives are typically Ag-filled and ceramic-filled epoxy based materials including flexible epoxies. They are medium to high modulus adhesives (>100,000 psi at room temperature). It is generally known that cured coatings of such materials have high intrinsic stress which can cause disruption of interface integrity due to delamination. This results in increased contact resistance with a corresponding decrease in the heat dissipation effectiveness at the interface. The commercially available Ag-filled adhesives also have no simple and practical rework method available. Therefore they cannot be readily removed or reworked from contacting surfaces. The non-reworkability of these adhesives present a serious drawback in that it does not allow for defect repair or component recovery, recycle or reuse of high cost semiconductor devices, heat sinks and substrates.
The most desired improvements in the thermal interface material properties include: ability to form thin bond line with uniform thickness across interfaces, low thermal impedance, low stress and compliant systems for interface integrity during device operation, stable interfacial contact resistance in T/H (temperature-humidity) and T/C (thermal cycling), TCR stability (temperature coefficient of resistance), and reworkability for defect repair and reclamation of high cost module components. The preferred materials should also be amenable to removal from contacting surfaces to allow rework without causing any detriment to the module materials for defect repair, chip replacement, and recovery of high cost components, particularly special type heat spreaders having high thermal conductivity and chip joined modules for reclamation and reuse.
The ability to rework and recover components has become more important to recover production yield loss, reduce waste, and provide cost reduction in the fabrication of advanced technology high performance electronic products. Commonly used high thermal conductivity heat spreader materials include AlSiC, SiC (k=270), SiSiC (k=210), AIN, Al, Cu and other special types having low thermal expansion and high thermal conductivity, for example, CuW, diamond-SiC, surface metallized diamond such as with Ni, Cr, CrNiAu, diamond-like carbon etc., to confer other desirable properties as corrosion resistance and adhesion improvement. With the use of high cost diamond based heat spreaders which have the highest thermal conductivity of all other common type of heat sinks employed, having a rework option for the cured conductive films offers a major benefit of recovery/reclamation and reuse, thus providing a cost effective way to obtain significant increase in heat dissipation capability with the use of high thermal conductivity cooling element in conjunction with a thermal interface adhesive.
In view of the limitations in the use of conventional interface materials, there is a need for improved thermal interface materials (TIMs) with efficient heat dissipation from high power density devices. There is also a need for a practical method to rework the cured deposits/residue of these materials from various component surfaces/interfaces the materials are adhered to.